Note: Descriptions are shown in the official language in which they were submitted.
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CONE-BEAM CT HALF-CYCLE CLOSED HELICAL TRAJECTORY
DESCRIPTION
The following relates to medical imaging systems. It finds particular
application to computed tomography (CT) and, more particularly to a CT imaging
approach for acquiring substantially complete sampling of a volume of interest
(VOI)
with efficient use of the radiation detectors.
With cone-beain CT, a complete data set or sampling of a VOI can be used to
reconstruct the VOI. Conventional cone-beam CT scanning techniques that employ
a
circular (or axial) radiation source orbit (path, trajectory, etc.) around an
imaging
region fail to provide complete sampling. Instead, the resulting data set is
not
complete in that the sampling on either end to the sample VOI is incomplete.
One
approach for obtaining complete sampling with the circular orbit is to perform
circle
and/or line scans, and then combine the scans together.
In an alternative approach, a saddle radiation source orbit is used to achieve
complete sampling of the VOI. Such a saddle orbit is described in
"Investigation of a
saddle trajectory for cardiac CT imaging in cone-beam geometry," Pack et al.,
Phys.
Med. Biol., vol. 49, No. 11 (2004) pp. 2317-2336. However, with the saddle
orbit the
width of the radiation detector in the z-direction is relatively larger than
the detector
width used with the circular orbit. This is due to a larger source trajectory.
Thus,
with the saddle orbit, complete sampling can be achieved at a cost of reduced
detector
efficiency. The increased detector size may lead to an overall increase in the
cost of
manufacturing the CT system since the radiation detectors account for a
relatively
large percentage of the total cost of such system.
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In view of the above deficiencies with conventional techniques, there is an
unresolved need for improved techniques for acquiring complete sampling of a
VOl
used to reconstruct the VOI while mitigating these as well as other
deficiencies.
According to one aspect, a tomographic apparatus having radiation source, at
least one radiation sensitive detector, and a reconstruction system is
illustrated. The
radiation source sweeps along a z-axis and returns to its initial position in
coordination with about two revolutions of the radiation source about an
imaging
region with a frequency of about half a frequency of a revolution of the
radiation
source about the imaging region. The at least one radiation sensitive detector
detects
radiation emitted by the radiation source that traverses a volume of interest
within the
imaging region and generates data indicative of the detected radiation. The
reconstruction system reconstructs the detected data to generate an image of a
subject
in the volume of interest.
Figure 1 illustrates an exemplary medical imaging system with a radiation
source orbit that at least efficiently utilizes the width of the radiation
detectors.
Figure 2 illustrates an exemplary half-cycle closed helical (HCCH) radiation
source orbit over two gantry revolutions.
Figure 3 illustrates the HCCH orbit along the z-axis as a function of gantry
rotation angle in degrees.
Figure 4 illustrates an exemplary profile of the HCCH path along the-z axis.
Figure 5 illustrates a method for scanning a subject using a HCCH radiation
source orbit.
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With reference to Figure 1, a medical imaging system 10 is illustrated. The
medical imaging system 10 uses various techniques to acquire suitable data of
a
region or voluxne interest of a subject and image slices, multi-dimensionally
rendered
or other of the region or volume of interest therefrom while making efficient
use of
the detectors. For instance, the medical imaging system 10 can employ one or
more
different x-ray source orbits, paths, trajectories, etc. around the subject
while
irradiating the subject and detecting transmission, scatter, etc. radiation.
Examples of
such orbits include a circular (or axial), a circular/line, a saddle, an
ellipse, a helical
segment, and/or a half-cycle closed helical, and/or derivations thereof,
and/or other
orbital paths.
Using the half-cycle closed helical, the medical imaging system 10 can acquire
a complete data set, or complete sampling (e.g., in a single non-discontinuous
motion), of a volume of interest (VOI) for generating images of the VOI (e.g.,
via 180
degree reconstruction, etc.). Such acquisition can be achieved while
efficiently using
the detectors (e.g., minimizing detector width along the z-axis, etc.),
reducing the
number of gantry revolutions (and thus, scan time and/or patient dose), and/or
increasing ease of repeatability.
As depicted, the medical imaging system 10 includes a CT scanner 12. The
CT scanner 12 includes a rotating gantry 14, which rotates about a z-axis 16.
The
rotating gantry 14 supports one or more x-ray tubes 18, which generate at
least one
radiation beain (e.g., conical, fan, etc.) at one or more positions, such as
focal spots, of
one or more radiation sources 20. One or more of the focal spots may be
dynainic in
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that they can rapidly shift or deflect to a plurality of positions during
rotation of the x-
ray tube 18 around the gantry 14.
In one instance, the radiation source 20 is movable along at least the z-axis
16.
Such movement can be achieved by physically or mechanically moving the x-ray
tube
18 along the z-axis and, hence, sweeping the radiation source 20 along the z-
axis 16,
and/or electronically by deflecting the x-ray tube I S electron beam so that
it impinges
the anode to the x-ray tube 18 at various positions along the z-axis 16.
1'bysical
movement of the x-ray tube 18 and radiation source 20 along the z-axis 16 is
coordinated with the rotational movement of the gantry 14 to provide a desired
radiation source orbit or trajectory. A gantry orbit bank 22 stores the
orbital paths for
the scanner 12. As depicted, suitable orbits include a half-cycle closed
helical
(HCCH) orbit 24, which will be described in greater detail below and,
optionally,
other orbits.
The gantry 14 also supports x-ray sensitive detectors 30 disposed about the
gantry 14 to subtend an angular arc opposite the x-ray source 18 to define an
imaging
region 32 therebetween. As depicted, the detectors 30 are arranged in a third
generation configuration. However, other detector arrangements, including
fourth
generation, stationary source systems, e-beam scanners, and/or other system
geometry
arrangements, are also contemplated herein. Each of the detectors 30 includes
one or
more single or multi-slice regions. The detectors 30 detect radiation emitted
by the
radiation source 20 that traverses the imaging region 32 and generate
corresponding
output signals indicative of the detected radiation along a plurality of rays.
The CT scanner 12 further includes a subject (or patient) support 34 that
supports a subject within the imaging region 32. The support 34 may be
stationary or
movable along x, y, and/or z-axes. Such movement allows an operator to guide
the
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subject to a suitable location within the imaging region 32 by moving the
support 34
or the support 34 in coordination with the gantry 14 (e.g., tilt, z direction,
etc) so as to
generate a desired scanning trajectory or orbit.
A computing system 36 facilitates operator interaction with and/or control of
the scanner 12. The computing system 36 can be a computer such as a
workstation, a
desktop, a tower, a laptop, or the like. In one instance, the computing system
36 is a
separate general-purpose system that executes applications and/or includes
hardware,
firmware, and/or software for communicating with the scanner 12. In another
instance, the computing system 36 is a dedicated console for the scanner 12.
Software applications executed by the computing system 36 allow the operator
to configure and/or control operation of the scanner 12. For instance, the
operator can
interact with the computing system 36 to select scan protocols, initiate,
pause and
terminate scanning, view images, manipulating volumetric image data, measure
various characteristics of the data (e.g., CT number, noise, etc.), etc. The
computing
system 36 communicates with a controller 38 that controls the scanner 12 based
on
the scan parameters. Such communication may include conveying computer
readable
instructions to configure and/or control the scanner 12 for a particular scan
protocol.
For example, such instructions may include parameters such as x-ray tube
voltage and
current, radiation source and x-ray tube position, radiation source orbit,,
etc.
Data collected by the detectors 30 is conveyed to a reconstruction system 40
that reconstructs the data to generate volumetric data indicative of the
scanned region
of the subject. The reconstruction systein 40 can be a dedicated system for
the
scanner 12 and/or a separate general-purpose computer. In addition, the
reconstruction system 40 may be an integrated and/or distributed system,
wherein
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subsystems (not shown) such as, but not limited to, a convolver, a
backprojector, etc.
are part of the saine system or distributed over separate subsystems or
computers.
An image processor 44 processes the volumetric image data generated by the
reconstruction system 40. In one instance, the image processor 44 generates
images
of the scanned anatomy that are displayed, filmed, archived, forwarded to a
treating
clinician (e.g., emailed, etc.), fused with images from other imaging
modalities,
further processed (e.g., via measurement and/or visualization utilities and/or
a
dedicated visualization system), stored within the storage component 42, etc.
Figure 2 illustrates an exemplary orbit, path, trajectory, etc. for the
previously
defined HCCH orbit 24 over two gantry revolutions. In Figure 2, the radiation
source
of the x-ray tube 18 moves along the z-axis 16 and follows a helical orbit 48
as the
gantry 14 rotates around the imaging region 32. The radiation source 20 sweeps
along
the z-axis 16 in both directions in a continuous motion such that the
radiation source
20 returns to its initial position (or closes the helix) after two gantry
rotations, or 720
15 degrees. Thus, the radiation source 20 helically tnoves through half a
cycle of motion
with each gantry rotation, or 360 degrees, and closes after two gantry
rotations.
The periodicity of the HCCH orbit 48 renders mechanical based radiation
source sweep implementations (e.g., via physical movement of the x-ray tube 18
)
relatively more feasible than with other orbital paths like the saddle orbit
since the x-
20 ray tube 18 can be moved at a relatively slower rate. Various hardware
and/or
software techniques can be used to compensate for acceleration and/or velocity
differences of the movement of the x-ray tube 18 along the-axis 16.
Figure 3 illustrates the HCCH orbital path 48 along the z-axis 16 as a
function
of gantry rotation angle in degrees over 720 degrees. In Figure 3, the path 48
is
shown as a smooth continuous function (sinusoidal); however, other paths,
though not
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preferred, such as discontinuous, triangular, etc. are also contemplated
herein. As
described in more detail below, complete sampling of the VOI is achieved with
data
collected over about one and one quarter revolutions, or about 450 degrees.
Figure 4 illustrates an exemplary profile 50 of the radiation source 20
following the HCCH path 48 along the-z axis 16 over two gantry revolutions
(e.g.,
starts at 58 or 60 travels 360 degrees to 60 or 58 and then returns over 360
degrees
back to 58 or 60). For complete sampling of a cylindrical VOI 52, the
radiation
source trajectory 48 encloses the VOI 52 as shown. An approximate extent (or z-
axis
width) of each of the detectors 30 for acquiring the VOI 52 is defined by rays
54 of
the x-ray beam and illustrated at 56. The source trajectory 48 makes efficient
use of
the detector along the z-axis 16, for a 180 degree reconstruction, of
substantially all
voxels within the VOI 52. By way of non-limiting example, using the HCCH
source
trajectory 48 for a 120 mm long VOI, the spot sweep is about 226.5 mm with a
detector extent of approximately 210 mm. Using a saddle trajectory for the
same
VOI, the detector extent would increase to about 328 mm.
As the radiation source 20 moves through a cycle and returns to its starting
position after two gantry revolutions, radiation is detected by the detectors
30. All or
a subset of the 720 degrees worth of detected data is used to reconstruct an
image(s).
For instance, each voxel can be reconstructed from at least 180 degrees plus
fan angle.
In order to reconstruct all voxels in the VOI 52, a subset of data collected
from about
one and one quarter revolutions is used. Thus, when performing a 180 degree
reconstruction, the reconstruction system 40 uses a suitable portion of
detected data
collected over two gantry revolutions to reconstruct images. That is, a
desired subset
of the data collected over two revolutions may be selected for reconstruction.
In
another instance, since 720 degrees worth of data is not required for a 180
degree
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reconstruction, the x-ray tube 18 can be turned off after enough data is
collected for
reconstruction.
Upon acquiring the data, a voxel-dependent 180 degree reconstruction can be
performed to image the VOI 52. After parallel rebinning of the projections, pi-
surfaces can be identified that intersect the VO1 52 at a given pair of source
angles.
Voxels at the intersection are reconstructed using the 180 degree range of
views
between the pair of source angles. For each 180 degree reconstruction, more
than 180
degrees plus fan angle worth of data can be used, if desired, to minimize
motion
differences at the beginning and the end of each subset of data. For example,
overlapped data acquired at different times can be averaged.
Figure 5 illustrates a non-limiting method for scanning a subject with the
medical imaging system 10. At reference numeral 62, an operator interacts with
scanner software applications executed by the computing system 36 to configure
and/or control operation of the scanner 12 to scan a subject in the imaging
region 32.
For this example, assume the operator has selected a 180 degree reconstruction
technique, either directly and/or indirectly through selecting a scan
protocol, etc, that
uses 180 degree reconstruction. Also assume that for the selected procedure
the
radiation source 20 is moved along the z-axis 16 by mechanically moving the x-
ray
source 18 (e.g., physical movement) and/or electronically sweeping the
generated
beain through the HCCH orbit 24 (which is stored in the gantry orbit bank 22).
As
described above, using the HCCH orbit 24 the radiation source 20 sweeps along
the z-
axis 16 in both directions in a continuous motion such that the radiation
source 20
moves through and closes a helix path (or returns to its initial position)
after two
gantry rotations. The computing system 36 communicates this and other
information
to the controller 38.
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At reference numeral 64, the control system 38 conveys control commands,
which include instructions and/or parameters for moving the radiation source
20
through the HCCH orbit 24 to the scanner 12. As described above, radiation
source
20 movement is achieved by mechanical and/or electronic techniques. At 66, the
scanner 12 operates under the control commands and the radiation source 20 is
moved
through the HCCH orbit 24 while generating an x-ray beam. At 68, the detectors
30
detect the emitted radiation and produce signals indicative thereof. At 70,
the
reconstruction component 40 reconstructs the signals, based on the selected
180
degree reconstruction technique, and the image processor 44 processes the
reconstructed data to generate corresponding images. As described above, all
or a
subset of the 720 degrees worth of data is used to reconstruct images. About
one and
a quarter gantry revolutions worth of data provides a complete set of data for
reconstructing the images. The images can be stored in the storage component
42
and/or provided to the computing coinponent 36 for visual observance by the
operator, filmed, further processed, etc.
The systems and/or methods described herein and/or derivations thereof can
be used with low, mid, and/or high end systems, including applications for
partial
and/or whole organ imaging such as the heart, perfusion imaging of the heart,
brain,
etc., as well as other applications.
The invention has been described with reference to the preferred
embodiments. Of course, modifications and alterations will occur to others
upon
reading and understanding the preceding description. It is intended that the
invention
be construed as including all such modifications and alterations insofar as
they come
within the scope of the appended claims.
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